Tricolor fluorescent dot patterns are formed on the inner surface of a color cathode ray tube (hereinafter referred to as "CRT") by means of lithography. The light exposure in this lithography is conducted using a light exposure device. In this light exposure device, the exposure light ray is made to assume the same path as the path which the electron beam will assume when the manufactured CRT is in operation.
FIG. 3 is a simplified frontal view of a typical light exposure device for the manufacture of color CRTs. As illustrated, a support frame 1 comprises a left support wall 1a and a right support wall 1b. Support platforms 2a, 2b, 2c, and 2d are connected to support frame 1 at specific vertical spacings. A light source 4 is secured to the top of support platform 2a through a mounting fixture 3. Inside of a case 4a of a light source 4 is a linear light source 5, such as a linear very-high-pressure mercury lamp, and a light-transparent slit 7, having a regulator window 7a that regulates a radiation angle of an exposure light ray 6' radiated from linear light source 5.
A light intensity correction filter 8 is mounted on support platform 2b.
A pair of compound corrective lenses 9 and 10 are mounted on support platforms 2c and 2d, respectively, in such a way that the positions of these two lenses may be adjusted. The corrective lenses 9 and 10 have a function of deflecting light and serve to make the path of the exposure light ray as close as possible to the path of the electron beams of the manufactured CRT.
A color CRT face panel 11 is placed, inside-surface down, and substantially at the center of support platform 2e. A photo-sensitive layer 12 on the inside of face panel 11 is created by first applying a slurry containing a fluorescent substance dispersed in a photo-sensitive resin and then drying. A shadow-mask 13 is placed at a specific distance from and in opposition to photo-sensitive layer 12. Shadow mask 13 is detachably fit on face panel 11.
In the above configuration, the radiation angle of exposure light ray 6 from linear light source 5 is regulated at regulator window 7a of light-transparent slit 7. Light intensity is corrected at light intensity filter 8. Furthermore, exposure light ray 6 is corrected to coincide with a path of an electron beam by compound corrective lenses 9 and 10, passes through shadow mask 13, and strikes photo-sensitive layer 12 on the inside surface of face panel 11. Photo-sensitive layer 12 is thereby exposed.
As the compound corrective lenses 9 and 10, lenses having multiple lens elements, each of which has a single continuous surface, are used. These compound corrective lenses 9 and 10 thereby have compound surfaces. More specifically, the compound lens has its surface divided into a plurality of regions, and in each region the surface is continuous, either flat or curved. Along the boundaries between adjacent regions, the surface is discontinuous and there are steps at these boundaries. Each of the regions is formed of a lens element having a single continuous surface which is either flat or curved.
FIG. 4 shows an area of a prior-art compound corrective lens used in a light exposure device as described above. In the illustrated example, a lens element 10a' toward the outside (farther away from center of the compound lens) is thicker than an adjacent lens element 10b' toward the inside. The exposure light beam 6 is thereby blocked by a light absorbing layer 15 formed on the step at the boundary between the lens elements 10a' and 10b' (the light absorbing layer 15 is formed to avoid emission of a light ray which has passed through (and is refracted at) the surface of the step). As a result, a relatively large unexposed region 12a is formed on photo-sensitive layer 12. For this reason, toward the outer boundary of each lens element (i.e., the boundary of each lens element with the adjacent lens element on the outer side), there is an area which is not utilized for the correction of the path.
Thus, the effective surface area of the lens element is reduced. This limits design flexibility (degree of freedom), requires the use of lens elements having curved surfaces that may be hard to manufacture, and makes setting of lens elements more difficult.